Catheter devices having an inflatable balloon mounted at the distal end are useful in a variety of medical procedures such as coronary angioplasty, stent delivery and placement for the opening of occluded or blocked blood vessels, for urological and reproductive surgeries, and to deliver biologically compatible fluids, such as radiologically opaque fluid for contrast x-rays to precise locations within the body. These procedures often involve the insertion of the device into blood vessels of extremely reduced diameter for long distances through the vascular system. These applications require thin walled high strength balloons of a relatively inelastic or non-compliant nature that have predictable inflation properties.
Once the balloon is positioned at the desired location, it is inflated by supplying liquid under pressure through an inflation lumen to the balloon. The inflation of the balloon causes stretching of a blood vessel, for instance, to reestablish acceptable blood flow through the blood vessel.
Balloon catheters are therefore produced from materials that can withstand high pressure, even at very low film thicknesses. The strength of the material is determined by measuring the tensile strength, and films with high strength relative to film thickness are chosen. Examples of materials useful in balloon catheters include polyethyleneterephthalate (PET); polyether-polyester block copolymers such as the Hytrel® series of block copolymers, also referred to as thermoplastic polyester elastomers, available from Du Pont in Wilmington, Del. or the Arnitel® series available from DSM, the Netherlands, such as Arnitel® 540; and polyamide/polyether/polyesters such as PEBAX® 6333, 7033 and 7233. However, films produced from such materials tend to be harder, and to be scratch and puncture sensitive.
Scratches, abrasions, and even punctures can occur during handling and storage of the devices, or during use. Stents or other objects may scratch or puncture the balloon. Friction between the device and the vessel through which it is being passed can result in failure of the balloon at the weakened points that result from scratches, abrasions or punctures. Lubricious coatings can reduce the friction between the device and the vessel wall, but provides only limited protection and does not really address the problem of scratches, abrasions and punctures. These coatings improve the success rate by altering the coefficient of friction between the device and the vessel wall, but do not address the scratch and puncture resistance other than by building film thickness.
Balloon failure at points of abrasions, scratches or punctures can be a problem during inflation. The balloon may prematurely burst, or the point at which the abrasion, scratch or puncture is located tends to be weaker, and when inflated, will have a tendency to over expand at that point, leading to over extension or bulging in the balloon wall at the weakened point. These bulges can in turn cause damage to blood vessels, for instance. Over inflation is also a problem during stent delivery.
More compliant materials tend to be more scratch and puncture resistant, but do not provide the strength required to withstand the pressures used in some of these procedures. Non-compliance which is the ability to resist expansion beyond a predetermined size upon pressure, and to substantially maintain a profile, is required for balloon catheters, especially those utilized in small vessels. Excessive expansion of more compliant materials can result in the rupture or dissection of blood vessels.
There is a continuing need in the medical device area to provide improved protective layers or coatings to balloons formed of noncompliant materials to increase the resistance of such balloons to scratches, abrasions and punctures. These improved coatings provide increased resistance by building film thickness, but they do not address the issue of what occurs in the event that the balloon is scratched, abraded or punctured.
Efforts have been made to coat a balloon with a continuous coating of a thin durable material. The problem associated with such continuous coatings in that if a pinhole is present in the underlying balloon material, as the balloon is inflated, the coating will also inflate, and at the point of the pinhole, can separate from the balloon material and form a bubble which can dissect an artery or vessel.
Specific examples of such problems occur with oriented PET which is commonly used for forming catheter balloons by a stretch blow molding method. The PET can exhibit pinholes that emit a high-velocity jet of inflation fluid during inflation. This then will result in the bulge forming in the outer coating layer which can cause artery dissection. Pet also exhibits low tear resistance and does not take a crease.
One such method of improving the scratch or abrasion resistance of medical apparatuses is found in U.S. Pat. No. 5,766,158 issued Jun. 16, 1998 to Opolski which describes a protective surface coating for a medical device which contains a matrix polymer and a reinforcing agent to decrease the sensitivity of the medical apparatus to injuries, such as scratches, punctures, and the like. The reinforcing agent is lamellar, platelet, or fiber-like in structure and has a higher surface hardness that the surface hardness of the medical apparatus.